1. Field of the Invention
This invention relates to a heated glazing panel and to a control circuit for use therewith.
There is presently a growing demand for heated glazing panels. These heated glazing panels are used for example as heated windscreens in motor vehicles, locomotives, metro trains, aeroplanes etc. as heated rear windows in vehicles or as heated glazing panels for buildings. They may particularly be used for removing steam, moisture or frost which hinders the visibility through the panel. They may also be used as an additional source of heat or for eliminating cold wall effects and improving the comfort of the occupants.
The present invention concerns heated glazing panels which comprise a sheet of glass on one face of which is disposed a thin coating of conductive transparent material which constitutes a heating element for the panel when the coating is connected to a source of electrical power. This coating is for example formed of ITO (indium/tin oxide) or by SnO.sub.2 doped with fluorine, deposited by pyrolysis for example, or by a layer based on silver or gold deposited for example by cathodic vacuum sputtering (for example Bi.sub.2 O.sub.3 /Au/Bi.sub.2 O.sub.3 or SnO.sub.2 /Ag/Ti/SnO.sub.2 coatings).
2. Description of the Related Art
It is known from British patent GB 1420150 (Glaverbel-Mecaniver) to provide a control circuit for a heating element incorporated in a hollow glazing unit, the control circuit including an adjustable resistor according to the setting of which the temperature control range of the panel is determined, preferably in response to a temperature sensor. Such a circuit however is unable to adapt for sources of electrical power of differing nature. If the power supply differs from that for which such a control circuit was specifically designed, the heating power output of the glazing panel may be insufficient and the desired temperature range may not be achievable.
Depending upon the dimensions of the panel and the voltage and nature of the electrical power supply, it is necessary to adapt the total electrical resistance of the heating element in order to obtain the desired electrical power which corresponds to the required effect.
It is not economically profitable to adapt the construction to modify the specific resistance or the thickness of the coating according to specific needs for each application and to each form of the panel, for example the dimensions thereof. Further, it is difficult in certain cases, to find a compromise between the desired optical properties of the panel and the specific electrical resistance of the coating.
It has been proposed that in order to adapt the heating output of a given glazing panel to the nature of the electrical power supply to which it is to be connected, that the coating be cut according to a predetermined scheme, thereby to change the electrical resistance thereof.
It is however difficult to remove the coating and cut it in a manner to achieve a sufficient interruption to electrically isolate two portions of the coating without risk of failure of the isolation over time, and without the cut portion being visible. In general, this process leads to a fine band of slightly different colour, which may be situated in the visibility zone of the vehicle driver and may distract his attention. It is in any case unattractive. This cutting away of the coating also necessitates an added cost in the construction of the panel.